Nitride semiconductors are investigated with respect to application to semiconductor devices with high withstand voltage and high output by using their characteristics such as high saturation electron speeds, wide band gaps, etc. For example, nitride semiconductor GaN has a band gap of 3.4 eV larger than the band gap (1.1 eV) of Si and the band gap (1.4 eV) of GaAs and has high breakdown electric field strength. Therefore, nitride semiconductors such as GaN and the like are very promising as semiconductor device materials for power supplies from which high-voltage operation and high output are obtained.
There are many reports on field effect transistors, particularly high electron mobility transistors (HEMT), as semiconductor devices using nitride semiconductors. For example, AlGaN/GaN HEMT using GaN as an electron travelling layer and AlGaN as an electron supply layer attracts attention as GaN-based HEMT (GaN-HEMT). The AlGaN/GaN HEMT causes distortion in AlGaN due to a lattice constant difference between GaN and AlGaN. High-concentration 2DEG (Two-Dimensional Electron Gas) is produced by a difference between the resulting piezoelectric polarization and spontaneous polarization of AlGaN. Therefore, the GaN-HEMT is expected as a high-efficiency switch element and a high-withstand-voltage power device for electric cars and the like. In addition, from the viewpoint of circuit design and safety, the realization of nitride semiconductor transistors having normally-off characteristics is desired.
The above-described semiconductor devices using compounds semiconductors such as nitride semiconductors are desired to have low on-resistance during transistor operation.
The followings are reference documents.                [Document 1 ] Japanese Laid-open Patent Publication No. 2012 -151422,        [Document 2 ] Japanese Laid-open Patent Publication No. 2012 -9630 and        [Document 3 ] Japanese Laid-open Patent Publication No. 2008 -124373.        